Integrated circuits have had almost universal application to communication and military technologies for several years. Of increasing importance has been development of microcircuit wafers and methods for interconnection of the circuits by automated equipment. A primary limitation to application of microcircuit technology has been cost efficiency and reliability of interconnection of integrated circuits on chips because of the small size of the chips, which often require hundreds of connections to be made within each circuit.
One method of circuit interconnection is called flip chip bonding. Flip chip bonding can offer a shorter signal path and, therefore, more rapid communication between circuits than can other methods, such as tape automated bonding (TAB) or conventional wire bonding, because bond pads on flip chips are not restricted to the periphery of the chip, but rather are usually located at one face of the chip opposite a substrate. In one method of flip chip bonding, a chip or die is formed with the requisite integrated circuit and interconnect wiring required for interconnecting the circuit with other chip circuits on a circuit board, such as a separate printed circuit board or substrate. Bond pads are located at points of interconnection. Bumps are formed by plating of several layers of metals on the bond pads of the flip chip. Following deposition, the chip is heated to reflow the metals, thus causing surface tension of the deposit to form hemispherical solder "bumps." The flip chip is subsequently severed from the wafer of which it was a part and "flipped" for alignment with the bond pads of a substrate. These bumps are then contacted with the bond pads of the substrate and uniformly heated to simultaneously form interconnects between aligned bond pads of the flip chip and the substrate.
Use of metals to interconnect bond pads of flip chips and substrates has required, however, that passivation of the flip chip be accomplished by use of a metal barrier such as titanium (Ti), tungsten (W) or silicon nitride (Si.sub.3 N.sub.4). Both the metal, as a passivation (or barrier) material, and ceramic, as a substrate material, are generally necessitated to allow sufficient heating to enable reflow of the solder bumps for interconnection between the flip chip and the substrate without consequential damage to either.
Fabrication of circuits using bumped flip chips have also been limited by the inability to visually inspect interconnections between the flip chip and the substrate. Further, the yield of finished mounted circuits can be detrimentally affected by failure of interconnects caused by the difference between the coefficients of thermal expansion of the various materials comprising the flip chip, the passivation layer, the solder bumps and the substrate. Also, melting of the solder bumps creates an electrically conductive flux as an undesirable byproduct which generally must be removed from between the substrate and the flip chip to allow proper operation of the finished circuit.
Problems of heat stress during fabrication have been addressed by various methods, such as by rapid application of heat to a bumped flip chip and rapid conduction of heat from the solder interconnects in order to minimize damage to flip chips, substrates and interconnections due to internal stresses caused by thermal expansion and contraction. However, this method is very expensive.
Therefore, a need exists for a method of interconnecting flip chips to substrates which is fast, cost-effective and reliable, so that the advantages of flip chips over other types of microcircuit wafers can be exploited more fully. Also, there is a demand for a simplified method of connecting flip chips to substrates which eliminates the need for elaborate plating procedures. Further, a method which enables greater flexibility of passivation and choice of substrate is also desirable. These improvements could promote cost efficiency and broaden the applications for which microcircuits are suitable.